1. Field of the Invention
This invention relates to improvements in high performance cavity-down, ball grid array HBGA packages for integrated circuits.
2. Prior Art
A cavity-down HBGA package mounts an integrated-circuit die inside a cavity formed through the bottom side of a die-carrier/heat sink. Bonding wires are connected between wire-bonding pads on the integrated-circuit die and wire-bonding sites on an insulated polyimide tape layer, where the wire-bonding sites on the tape layer are located outside of the cavity and adjacent to the cavity on the bottom side of the die-carrier/heat sink.
The integrated-circuit die, the bonding wires, and the conductive bonding sites on the insulated tape layer are all encapsulated in an encapsulation cover. The encapsulation cover extends below the plane of the insulated tape layer. If the encapsulation cover is too thick, it may interfere with mounting of the HBGA package to a component-mounting surface of a printed-circuit board.
A number of conductive traces are formed on the insulated tape layer to connect the conductive bonding sites to selective solderable areas for solder balls. The selective solderable areas are located on the insulated tape outside of the cavity and away from the encapsulation cover. The selective solderable areas for the solder balls are formed on the insulated tape layer and are arranged in a grid pattern on the bottom side of the die-down HBGA package. A solder mask is formed over the insulated tape layer but not over the selective solderable areas on the insulated tape on the bottom side of the die-carrier/heat sink.
In order to mount an HBGA package to the surface of a printed-circuit board, the grid pattern of the solder balls is placed over a corresponding grid pattern of solderable areas on the printed-circuit board. The solder balls are then heated to a temperature sufficient to melt the solder balls and to solder the HBGA package to the surface of the printed-circuit board.
The bonding wires form wire loops. The wire loops extend between the bonding-wire pads on the integrated-circuit die, which are located in the cavity, to the bonding sites on the tape layer, which are located outside of the cavity. The bonding-wire loops also extend downwardly below the plane of the insulated tape layer on the bottom side of the die-carrier/heat sink. On the one hand, the encapsulation cap must be sufficiently thick so that it completely encapsulates the downwardly extending bonding-wire loops. One the other hand, the encapsulation cap must also be sufficiently thin so that sufficient clearance is provided between its bottom surface and the component-mounting surface of a printed-circuit board, when the HBGA package is soldered to the printed-circuit board with solder balls.
FIG. 1 illustrates a conventional HBGA package 10, which includes a die-carrier/heat spreader 12 which is formed of a metallic or ceramic material. The die-carrier/heat spreader has a die-cavity 14 formed through its lower surface. An integrated-circuit die 16 is attached to the top interior surface 18 of the die-carrier/heat spreader 12. A plurality of bonding wires are connected between the integrated-circuit die 16 and conductive traces formed on an insulated layer which surrounds the die-cavity 16. An encapsulation cap 20 covers the integrated-circuit die 16 and the bonding wires. A number of solder balls, typically shown as 22, are arranged in a grid and attached to the conductive traces formed on the insulated layer. The solder balls are also connected to a number of corresponding solder-ball attachment sites located on the top surface of a printed-circuit board 24. Note that thickness of the encapsulation cap 20 must be sufficiently thin and the solder balls sufficiently large, so that the bottom surface of the encapsulation cap provides sufficient clearance to the top surface of the printed-circuit board 24.
With reference to FIG. 1 and also to FIG. 2 in greater detail, the bonding wires, typically shown as 30, have one of their ends attached to bonding pads, typically shown as 32, on the lower surface of the integrated-circuit die 16. The other ends of the bonding wires are attached to wire-bonding sites, typically shown as 34, which are formed on an insulated tape layer 24. The insulated tape layer 24 is attached with an adhesive layer 36 to a lower surface 38 of the die-carrier/heat spreader 12. The lower surface 38 of the insulated tape layer 24 surrounds the cavity 14.
The encapsulation material, which forms the encapsulation cap 20, fills the cavity 14 and covers the integrated-circuit die 16, the bonding wires 30, and the wire-bonding sites 34, as illustrated. As illustrated in FIG. 2, the bonding wires 30 droop below the plane of the lower surface of the insulated tape layer 24. The encapsulation cap 28 must be thick enough to cover the drooping wire loops of the bonding wires 30 while still providing sufficient clearance to the top surface of the printed-circuit board 24.
A high performance ball grid array HBGA package has a grid with a center-to-center spacing, or pitch, between adjacent solder balls of 50 mils for solder balls with a final, assembled height of 19 mils. A solder ball with an initial height of 24-25 mils collapses to 19 mils when it is mounted to a package. When the package is finally assembled to a printed-circuit board, the solder ball will have a final height of 19 mils. The depth of the cavity 14 in the die-carrier/heat spreader 12 is 18 mils and the combined thickness of the adhesive layer 36 and the insulated tape layer 24 is 6 mils. The droop of the bonding wires loops below the plane of the lower surface of the insulated tape layer 24 is 10 mils. The distance w between the lowest point of a bonding wire and the lower surface of the encapsulation cap is 4 mils. For a solder ball having a thickness of 19 mils, the spacing d between the lower surface of the encapsulation cap 20 and the top surface of the printed-circuit board 24 is only 5 mils to provide clearance between the HBGA package and the surface of the printed-circuit board.
HBGA packages having solder-ball grid spacings smaller than 50 mils require using solder ball with final thicknesses less than 19 mils. Consequently the package configuration illustrated in FIGS. 1 and 2 is limited to larger solder balls and larger grid spacings.
Consequently, it can be appreciated that the design of an HBGA package requires attention to the dimensions and allowable tolerances for certain critical parameters, such as the droop of the wire loops and the thickness of the encapsulation cap. This is especially important for higher density cavity-down BGA packages with a solder-ball spacing, or pitch, which is less than 50 mils or 1.27 mm. pitches. As the diameter of the solder balls get smaller, the clearance between the package and the surface of the motherboard also get smaller.
Consequently, a need exists for an improved cavity-down HBGA package which minimizes the thickness of the encapsulation layer while still accommodating a greater number of bonding wires and which provides for a closer grid spacing for smaller solder balls.